[Bug middle-end/114700] Front-end optimization generates wrong code with -fsanitize=undefined

["lin1.hu at intel dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114700

--- Comment #12 from Hu Lin <lin1.hu at intel dot com> ---
(In reply to Hu Lin from comment #11)
> (In reply to Richard Biener from comment #9)
> > That that GCC doesn't promise that -ftrapv preserves all overflows and
> > traps, it merely guarantees that all overflows that actually happen trap. 
> > So GCC is fine to contract some expressions where the overall number of
> > overflows can only
> > decrease.
> > 
> > That's not a bug with -ftrapv.
> > 
> > It is considered a bug with -fsanitize=undefined though.
> 
> I think it doesn't mean that's not a bug with -ftrapv, it should preserve
> all overflow traps. Because it doesn't work, we use -fsanitize=undefined
> instead of it.
> 
> refer: Gcc's trapv is known not always to work correctly.
> 
> The current behavior is correct for -fsanitize=undefined, because the
> integer signed overflow is well-defined, so GCC can eliminate some
> variables. I just think GCC can optimize `z = c  - y  - c + a  + y - b` to
> `z = a - b`. But it doesn't mean is a bug for -fsanitize=undefined.

I was wrong about signed overflow, it's undefined behavior in c++20
(https://en.cppreference.com/w/cpp/language/ub). If I'm not mistaken about the
source， it's a bug.